1. Field of the Invention
This invention relates to the field of molecular beam epitaxy (MBE), and in particular to a method of achieving GaN epitaxial selective growth and lateral overgrowth on a substrate.
2. Description of Related Art
Gallium nitride epilayers are becoming increasingly important in the manufacture of a number of devices. GaN epilayers are, for example, the crucial technology used for achieving robust CW blue lasers with long lifetime. Selectively grown GaN stripes, prisms and pyramids with their characteristically smooth facets or well defined ridges are ideal candidates for forming optical microcavity, optical waveguides, and InGaN quantum dot structures.
The selective growth of GaN has been demonstrated using the metalorganic chemical vapor deposition (MOCVD) technique and successfully used in a variety of applications over the past four or five years. The epitaxial lateral overgrowth enabled by this selective growth process has been shown to bring a dramatic reduction by many orders of magnitude of the threading dislocation density in GaN epilayers.
MOCVD is however a complicated technique and it would be desirable to achieve selective growth by the molecular beam epitaxy (MBE) technique. This, however, has proved to be generally much harder than by vapor-phase-based growth processes such as the MOCVD technique. Gas phase precursors are generally much more selective to substrate materials in terms of adsorption, cracking and incorporation than simple atoms effused from an MBE cell. Earlier studies of GaN growth on patterned SiC substrates using plasma-source MBE, and on patterned Si substrates using ammonia-source MBE found no evidence of growth selectivity. However, a recent work by V. K. Gupta, K. L. Averett, M. W. Kock, B. L. Mcintyre, and G. W. Wicks J. Electron. Mater. 29, 322(2000) using ammonia-source MBE examined the difference in ammonia cracking efficiencies on various material surfaces and showed evidence of selective growth with a SiO2 mask. Also, MBE has in the past been associated with significant surface roughness, and this can preclude many possible device applications.
The quality of GaN materials grown by the ammonia-MBE technique has seen a significant improvement in the past few years, and is now comparable to the best-grown MOCVD layers. For example, this technique produced a GaN bulk layer on sapphire with a mobility of 560 cm2/Vs at 300 K (nxcx9c1.5xc3x971017 cmxe2x88x923), and an AlGaN/GaN layer on SiC with a record mobility of 11000 cm2/Vs at 77 K (nsxcx9c3.2xc3x971012 cmxe2x88x922). However, GaN epilayers prepared by this technique generally show a significant surface roughness associated with the sizes of the columnar structures defined by the density of threading dislocations, and these can compromise the performance of certAlN types of devices. Therefore, the characteristics of selective growth such as extremely flat facets and dramatic reduction in threading dislocations through lateral overgrowth would be highly desirable if feasible with the ammonia-MBE technique.
According to the present invention there is provided a method of selectively depositing a GaN epitaxial layer on a substrate, comprising patterning the substrate with a seed layer; and growing the GaN epitxial layer by molecular beam epitaxy (MBE) on the patterned substrate such that growth occurs selectively over the seed layer.
The substrate is typically SiC and the seed layer is preferably AlN. In accordance with the invention, excellent selective growth of GaN on a SiC substrate by ammonia-MBE has been observed. This occurs preferentially from the patterned AlN seed layer and uses the unseeded SiC surface as a pseudo mask.
The difficulty of GaN nucleation on SiC surface has been widely observed in the MOCVD processes. A similar and total absence of nucleation on a bare SiC substrate with the ammonia-MBE process was observed even after a three-hour growth period. This is in contrast to the nearly instant nucleation of GaN on sapphire substrates found under typical ammonia-MBE growth conditions. However, when the thin (a few hundred xc3x85) AlN seed/buffer layer is deposited first on the SiC substrate, the subsequent GaN growth occurrs instantly as could be seen by in situ laser reflectance measurement.
The AlN seed layer is preferably pre-deposited on the SiC surface using the magnetron sputter epitaxy techniqne (or MEE) and patterned into parallel stripes by photolithography and chemically assisted ion beam etching. Evidence of lateral overgrowth can be observed by scanning electron microscopy and x-ray diffraction studies. The GaN stripes grown show extremely smooth side facets due to the lateral growth.
The lack of gas diffusion may be offset by enhancing atoms"" surface mobility through optimizing ammonia flow rate and/or using certain surfactants during growth.